The invention is directed to a vacuum furnace for the heat treatment of steel, particularly for reduced pressure coking with a graphite heater.
Vacuum furnaces with graphite heaters such as are required for the heat treatment of steels are generally equipped with heating elements made of graphite rods, graphite tubes or of graphite cloth.
Graphite rods or tubes, i.e., heating elements made of solid graphite, have the disadvantage that they have a relatively small resistance. Therefore, there must be provided a long current path, frequently under great difficulties of construction in order to obtain the necessary resistance. Furthermore, the material introduced with the solid graphite is very large and therewith the thermal capacity also is very large, which in turn makes the furnance sluggish in the heating and cooling. A further disadvantage of solid graphite heating elements is the relatively small heating surface which is provided at a specified power so that there cannot be attained a large surface heating.
These disadvantages of solid graphite heating are avoided through the graphite cloth heating. This heating with a graphite fibrous tissue is distinguished above all else by a relatively large resistance, since it is very thin. It has a very small heat capacity and if offers a large heating surface. These clear advantages of graphite cloth, however, are opposed by disadvantages which in certain cases exclude their use as heating elements. In many cases there is incorporated in the vacuum furnace a gas circulator in order that after the thermal treatment is carried out the batch and furnace can be cooled down in accelerated manner under inert gas. For these cases the graphite cloth heating element is not suited because of its low mechanical strength since the cloth is easily damaged by the flow of gas.
A still greater problem is present if a graphite cloth heater is used in furnaces which also are used for reduced pressure coking.
In these processes there is formed atomic carbon by the destruction of the coking gas which carbon also deposits on the heater cloth. Since the cloth is relatively thin, the enrichment with carbon in a short time causes a strong change in the resistance characteristics of the cloth through which the resistance sinks so far that the necessary furnace output can no longer be attained because of the always present limitation on the current. Besides the cloth loses its flexible character and becomes brittle.
Therefore, it was the problem of the present invention to provide a graphite heater for vacuum furnaces which makes possible a large heating surface, has a small heat capacity, whose resistance characteristics are not substantially changed by depositing carbon and which is mechanically stable.